In gas production, based on gasification of biomass, coal, etc., it is well-known that the produced gas contains tar, which in connection with the use of the gas in e.g. internal combustion engines or gas turbines, has to be removed from the produced gas. The traditional way of removing the tar is by cooling the gas, whereby the tar and possible water is condensed, leaving a relatively clean gas and a separated mixture of water and tar.
The mixture of water and tar cannot be disposed of to the environment due to the content of tar, including polyaromatic hydrocarbons and phenols, which are considered environmentally unsafe, possibly carcinogenic, poisonous, etc. Furthermore, the acidity of the mixture may also constitute an environmental problem.
It has been suggested to use ultraviolet light-induced wet oxidation or adsorption on various coke sorbents to clean the waste water. However, electric energy consumption is relatively high for the ultraviolet light-induced wet oxidation and the adsorption has certain limitations due to the presence of non-absorbable compounds.
It has been suggested (Swedish application 402214) that the waste water is evaporated and separated into a combustible tar fraction and a (more or less) clean steam fraction. The combustible fraction is subsequently burned (oxidized) to provide heat for the evaporation process—either by direct contact or through a heat exchanger. However, in this way the calorific value of the tar is converted directly to heat, which severely limits the utilization in high efficiency power producing machinery—specifically gas engines and gas turbines. Therefore, the application of the technology suggested will be limited to steam turbines and other equipment based on external firing.